This invention relates generally to induced radiative effects such as Raman and fluorescence detection and, in particular, to methods and apparatus for automatically calibrating wavelength and intensity aspects of such a system.
Raman spectroscopy is gaining increasing acceptance in on-line process monitoring, due in large part to developments in instrumentation and associated component technologies. For a number of process applications, Raman analyzers have demonstrated significant advantages over alternative techniques such as gas chromatography, IR spectroscopy, and NIR spectroscopy. As a non-destructive, real-time technique, Raman spectroscopy is compatible with a wide variety of samples including opaque solids, aqueous solutions, emulsions, and gases, without the need for sample preparation.
The basic building blocks of a Raman instrument are shown in FIG. 1. An excitation laser 102 illuminates a sample under test 104. Scattered light 106 is collected by optic(s) 108. A notch filter 110 is typically used to remove the strong Rayleigh (unshifted) component that would otherwise hide the weak Raman signal. A spectrograph 112 spectrally separates the remaining Raman signal, which is then fed to a detector 114 coupled to processing hardware and software 116.
Sampling in a process environment is most conveniently accomplished using a probehead assembly, as shown in the prior-art system of FIG. 2. Delivery of the excitation laser beam to the sample under test 200 is accomplished via an excitation fiber-optic cable 202. Scattered light from the sample is collected by the probehead and routed back to the analyzer via a separate collection fiber-optic cable 204.
Typical process installations may run hundreds of meters of fiber cable between the analyzer and probehead. Each optical fiber has its own Raman and luminescence signatures, which must be removed at the probehead before the laser illumination is delivered to the sample. Similarly, the strong laser Rayleigh line must be removed from the collected frequency-shifted scatter at the probehead before entering the return fiber; otherwise, it may generate a competing fiber signature on the way back to the analyzer. A holographic grating 206 and spatial filter 208 are therefore inserted into the beam delivery path to remove the fiber signature. The Rayleigh line is removed from the collected Raman scatter by a holographic notch filter 210 in the collection path. A holographic beam combiner 212 serves to combine the laser beam delivery path onto a common optical axis with the collection path, so that a common sampling optic 214 may be used for both paths.
As shown in FIG. 3, modern Raman instruments may also be configured to monitor multiple sample points in a process. In a typical industrial installation, multiple remote probeheads 302 are coupled to a central instrument 304 via fiber optic cables 306. The central instrument 304 typically houses a laser source 310, spectrograph 312, CCD detector 314 and control electronics 316. A sequencer or splitter 320 is used to multiplex the output of the laser source 310.
Widespread acceptance of Raman spectroscopy in chemical process monitoring requires accurate and timely instrument calibration. The three key parameters to be calibrated in a Raman analyzer are: 1) the spectrograph wavelength axis; 2) the system spectral response or intensity axis; and 3) the laser wavelength itself.
Wavelength Calibration
Wavelength calibration determines the wavelength versus pixel mapping function of the spectrograph/camera assembly. There are a number of known wavelength calibration sources applicable to Raman spectrometers. Atomic emission lines from readily available neon or argon lamps form convenient wavelength calibration sources. Neon is preferable in that it provides emission lines in close proximity to the common 785 nm and 532 nm laser lines used in process Raman. A neon emission can also provide reference lines near both edges of the CCD for gratings used in certain types of commercially available Raman analyzer equipment.
Wavelength calibration for later use in Raman data analysis is most accurate if the calibration signal is fed into the spectrograph via the same optical path that is used to collect Raman data. In addition, two factors make this impractical in the process environment. First, it is impractical to insert a neon lamp into the process pipeline; second, it is inconvenient to remove the probehead from the pipeline so that a calibration source may be placed in the collection path.
In addition, because Raman detection is a frequency-shift phenomenon, wavelength calibration of the spectrograph alone is not sufficient for analyzing Raman shifts with the greatest possible accuracy. Calibrating the wavelength or frequency of the excitation laser source is equally critical. While gas lasers such as helium-neon or argon-ion lasers emit precisely known atomic emission lines, the emission wavelengths of the solid-state lasers more common in process Raman are less stable, and therefore require frequent wavelength calibration.
Intensity Calibration
Intensity calibration determines the spectral response of the entire light collection path from the probe sampling optic to the CCD camera output signal. Intensity calibration in Raman spectroscopy is normally concerned with the relative spectral response of the system across its spectral range, rather than its absolute response. This is because sample composition or structure is normally determined from the shape of the Raman spectrum, such as band ratios, rather than an absolute response such as absorbance.
The purpose of intensity calibration is to correct the shape of the measured Raman spectrum. Factors that distort the shape of the spectrum as measured by the CCD include the spectral transmission of the optical elements, the spectral efficiency of the diffraction grating, the silicon quantum efficiency curve of the CCD detector elements, and pixel-to-pixel variations in CCD detector responsivity. By imaging a light source of known spectral output through the complete light collection path of the analyzer, all of these factors may be accounted for simultaneously.
In summary, although accurate laboratory calibration may be performed using external wavelength and intensity reference lamps, this approach requires access within or near the probehead assembly, which may impractical or unwarranted in a process environment. Although such access is not required when using internal calibration, existing techniques do not calibrate the intensity axis, and accuracy suffers because the actual optical collection path from a real sample is not used. The need therefore remains for a calibration arrangement which is conducive to automated, internal referencing while, at the same time, utilizes as much of the actual collection path as possible to enhance the accuracy of the system. A candidate configuration should feature an accuracy approaching that of the ideal laboratory protocol, without requiring lamps or electrical power at the process location.
This invention resides in methods and apparatus for calibrating remote optical probe configurations of the type wherein a spectrum emitted by a sample is delivered to a spectrograph for analysis. The teachings are applicable to various spectroscopic techniques, including fluorescence and Raman detection. Depending upon the embodiment, the system and processes may be used to calibrate the spectrograph wavelength axis, the system spectral response or intensity axis, and the wavelength of the laser used for excitation.
The preferred embodiment includes a collection optical fiber having a first end for receiving wavelengths emitted by the sample and a second end for delivering the wavelengths to a base unit containing the spectrograph. A calibration optical fiber is used to deliver an optical calibration signal to a point proximate to the first end of the collection optical fiber, and an optical element is used to direct the optical calibration signal into the collection optical fiber so that the optical analysis means receives both the wavelengths emitted by the sample and the optical calibration signal.
A neon reference lamp is preferably used to calibrate the spectrograph wavelength axis, whereas a tungsten halogen lamp is employed to calibrate the system spectral response or intensity axis. The wavelength of the excitation laser may also be calibrated using a known Raman scattering line of a standard material. The neon lamp may be placed directly against the calibration fiber such that the glowing neon gas emission will be directly in front of the fiber end. The tungsten halogen lamp is preferably used in a lensed configuration, such that the light is focussed through the neon lamp envelope onto the calibration fiber end. In the preferred embodiment, the laser and reference lamp or lamps are disposed at the same location as the base unit to simplify fiber routing.
The invention is applicable to a variety of configurations, including process monitoring environments in which a plurality of probeheads and collection optical fibers are used, each one being associated with a different sample or portion thereof. A plurality of calibration optical fibers are also used in such a case, each having one end for delivering calibration signals to each probehead assembly. To calibrate the laser wavelength, one of the probeheads may be assigned to receiving wavelengths emitted by a reference substance such as cyclohexane.